In some electronic systems implanted in the human body, power must be supplied from an outside device to the implanted device (or implant).
Said power supply is normally provided by inductively coupling a coil, called the transmitter and located outside the patient's body, to a coil called the receiver, which is implanted just below the patient's skin and is either connected to or incorporated into the implant.
The transmitter coil generates a field that is picked up and converted into electric current by the receiver coil when it is aligned with said transmitter coil.
The energy transferred to the implant can be exploited immediately or stored in a rechargeable battery for later use.
The outside device is hereinafter called the power supply, although it may equally operate as a battery recharger.
The effectiveness of the power transfer between the two coils is rather sensitive and depends mainly on the reciprocal positioning, i.e. on the coupling between the two coils.
The best performance is obtained when the two coils, the transmitter and the receiver, are perfectly aligned, but this alignment is not easy to achieve because the exact position of the receiver coil is difficult to identify.
Any displacement or misalignment between the two coils reduces the efficiency of the energy transfer, with a smaller quantity of energy consequently being transferred to the implant, and/or increases the time needed to recharge the implant's battery.
Moreover, a part of the energy delivered by the transmitter coil that is not transferred to the receiver coil is absorbed by the tissues surrounding the coils, giving rise to an increase in the temperature of said tissues.
Given the time required for the transmitter coil to recharge the implant's battery or to power the implant, exposing the tissues to the energy that is not absorbed by the receiver coil can have side effects of various types, ranging from mere discomfort to lesions with severe consequences.
Other known electronic implantable devices include a telemetric intercommunication system that enables the transfer of information from the implant to other, outside devices, such as a programmer, a function monitor, a power supply, a recharger, and so on. Such telemetric systems are used to record and to set the implant's working parameters, to record historical data on how the implant functions, and to receive and transmit other similar information.